Abstract

The glacial history of the westernmost Weddell Sea sector of Antarctica since the Last Glacial Maximum is virtually unknown, and yet it has been identified as critical for improving reliability of glacio-isostatic adjustment models that are required to correct satellite-derived estimates of ice sheet mass balance. Better knowledge of the glacial history of this region is also important for validating ice sheet models that are used to predict future contribution of the Antarctic ice sheet to sea level rise. Here we present a new Holocene deglacial chronology from a site on the Lassiter Coast of the Antarctic Peninsula, which is situated in the western Weddell Sea sector. Samples from 12 erratic cobbles and 18 bedrock surfaces from a series of presently-exposed ridges were analysed for cosmogenic 10Be exposure dating, and a smaller suite of 7 bedrock samples for in situ 14C dating. The resulting 10Be ages are predominantly in the range 80–690 ka, whereas bedrock yielded much younger in situ 14C ages, in the range 6.0–7.5 ka for samples collected from 138–385 m above the modern ice surface. From these we infer that the ice sheet experienced a period of abrupt thinning over a short time interval (no more than 2700 years) in the mid-Holocene, resulting in lowering of its surface by at least 250 m. Any late Holocene change in ice sheet thickness — such as re-advance, postulated by several modelling studies — must lie below the present ice sheet surface. The substantial difference in exposure ages derived from 10Be and 14C dating for the same samples additionally implies ubiquitous 10Be inheritance acquired during ice-free periods prior to the last deglaciation, an interpretation that is consistent with our glacial-geomorphological field observations for former cold-based ice cover. The results of this study provide evidence for an episode of abrupt ice sheet surface lowering in the mid-Holocene, similar in rate, timing and magnitude to at least two other locations in Antarctica.

Highlights

  • Ice streams flowing into the Weddell Sea Embayment (WSE) of Antarctica at the present day drain approximately a fifth of the Antarctic Ice sheet (Joughin et al, 2006)

  • We present exposure age data from a suite of rock samples collected from currently-exposed peaks and ridges of the Bowman and Smith Peninsulas on the Lassiter Coast of Antarctica (Fig. 1b)

  • This study presents the Holocene deglacial history of Johnston Glacier in the westernmost Weddell Sea Embayment determined from cosmogenic 10Be and in situ 14C exposure age dating of bedrock and erratic cobbles

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Summary

Introduction

Ice streams flowing into the Weddell Sea Embayment (WSE) of Antarctica at the present day drain approximately a fifth of the Antarctic Ice sheet (Joughin et al, 2006). Modelling studies suggest that changes in the current patterns of ocean circulation in the region over the centuries could bring warm ocean water into contact with grounding lines, resulting in increasing contributions to sea level rise from this sector of the ice sheet As a result of several marine and terrestrial geological campaigns studying the eastern and central regions of the WSE during the past decade — summarised in Hillenbrand et al, 2014, and subsequently Arndt et al, 2017; Balco et al, 2017; Bentley et al, 2017; Fogwill et al, 2014; Hein et al, 2014, 2016 — understanding of the configuration and extent of the Antarctic ice sheet in the WSE during the Last Glacial Maximum (LGM), as well as its subsequent thinning and retreat, has significantly increased. The westernmost margin of the WSE has seldom been visited; the pace and pattern of ice sheet change there since the LGM remains virtually unknown

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